1. Field of the Invention
This invention relates generally to antenna system design and more particularly to sidelobe blanking for use in radar motion compensation systems.
2. Description of Related Art
A Sidelobe Blanking (SLB) system is commonly used for mitigating interference in radar and telecommunications systems. As known in the art, the SLB system operates to compare the signal outputs of one or more low directivity auxiliary antennas to the output signal of the main antenna of the radar antenna system. When the signal level received in an auxiliary channel of an auxiliary antenna is equal to or greater than signal level in the main channel of the main antenna, the SLB system discards the main channel response as interference.
In certain applications, the SLB system is designed to provide near horizon coverage in a forward hemisphere for a limited scan phased array radar system. A phased array antenna system when mounted on a moving vehicle such as a ship may undergo significant three dimensional angular displacements relative to the horizon even in reasonably quiet seas, such as “roll”, “pitch” and “yaw”. Roll corresponds to the side-to-side angular motion about a longitudinal, i.e. fore and aft, axis of the vehicle. Pitch corresponds to the alternating motion about an axis perpendicular to the longitudinal axis of the vehicle. Yaw corresponds to the change in the azimuth direction about a vertical axis.
To compensate the effect of vehicle or ship motion, several approaches have been used. The mechanical rotation or servomechanism is commonly used in reflector antenna systems. Electronic beam steering of the main antenna beam is commonly used in phased array antenna systems. For the purpose of this discussion, electronic beam steering is defined as the ability to electronically steer the beam maximum of an antenna electric field pattern to some predefined point in space. Such electronic beam steering is performed on the main beam by altering the antenna electric field pattern during its transmission.
The combination of servomechanism and electronic beam steering is also used in many systems for economical and practical reasons. Most of the servomechanism motion compensation systems compensate for movement in two axes only—azimuth and elevation. It is because the radar beam shape used is a pencil beam shape. If a vertical fan beam is used, such as in the maritime surveillance radar SPS-49, the target altitude is unimportant and thus there is no need for motion compensation.
In the application of SLB against surface based (land or sea) interferences, a horizontal fan beam is used in the auxiliary antenna associated with the main antenna. When the platform of such SLB system moves, maintaining the roll angle of this horizontal fan beam is vital for the effectiveness of the SLB system. Such roll angle compensation is not known to exist in any radar system. One reason is that the use of a mechanical approach is difficult due to the mounting axle being inline with the radar beam. The alternative approach is electronic beam steering however; the number of antenna elements for the auxiliary antenna is usually very small. Hence, the capability of rotating such antenna pattern is very limited.